In recent years, as electrical devices such as smartphones have spread, an SRAM for processing a large-capacity signal such as an audio signal or an image signal at high speed has increased in importance (see, e.g., “Wikipedia. Static Random. Access Memory”, Internet (URL: http://ja.wikipedia.org/wiki/Static_Random_Access_Memory)).
Generally, in the SRAM, a higher speed, a smaller area, and lower power are important. In recent years, a new circuit configuration has also been developed. The SRAM is a volatile memory. Thus, external data written into a storage node desirably remains stored even after the supply of power is stopped, SRAM data is also desirably programmed into a non-volatile memory unit capable of storing data even after the power is stopped or desirably read out again into the storage node from the non-volatile memory unit after the power is turned on again.
However, in a general non-volatile memory unit, a voltage difference between a voltage value required during a programming operation for programming data and a voltage value required during a non-programming operation for not programming data is large. Therefore, a high-speed operation in an SRAM is difficult to implement because a voltage applied to the SRAM, which exchanges data with such a conventional non-volatile memory unit, also increases to match voltages required for the programming operation and the non-programming operation of data into the non-volatile memory unit, and thus the thickness of a gate insulating film of a transistor constituting the SRAM also increases.